Quantization in perceptual audio coders with compensation for synthesis filter noise spreading

Abstract

Many perceptual split-band coding systems that use analysis and synthesis filters assume the quantization noise introduced by quantizing split-band signals is substantially the same as the noise that results in the output signal obtained by applying the synthesis filters to the quantized split-band signals. In general, this assumption is not true because the synthesis filters modify or spread the quantization noise. A theoretical framework for deriving an optimum bit allocation that accounts for synthesis-filter noise spreading and the overlap-add process is disclosed. In concept, the problem of finding an optimal bit allocation can be expressed as a linear optimization problem in a multidimensional coordinate space. Simplified processes derived from this theoretical framework are disclosed that can obtain near-optimal solutions using modest computational resources.

What is claimed is:  
     
       1. A method for establishing quantization resolutions for quantizing subband signals obtained from analysis filters that are applied to an input signal, wherein an output signal that is a replica of the input signal is to be obtained by applying synthesis filters to dequantized representations of the quantized subband signals and by applying an overlap-add process to blocks of information obtained from the synthesis filters, the method comprising: 
       generating a desired noise spectrum in response to the input signal; and  
       determining the quantization resolutions for the subband signals by applying a synthesis-filter noise-spreading model to obtain estimated noise levels in subbands of the output signal obtained from the synthesis filters, wherein the synthesis-filter noise-spreading model represents noise-spreading characteristics of the synthesis filters and accounts for effects of the overlap-add process, and wherein the quantization resolutions are determined such that a comparison of the desired-noise spectrum with the estimated noise levels satisfies one or more comparison criteria.  
     
     
       2. A method according to  claim 1  that determines the quantization resolutions for the subband signals by a process that applies the synthesis-filter noise-spreading model to proposed quantization resolutions and adjusts the proposed quantization resolutions by a predefined amount of compensation. 
     
     
       3. A method according to  claim 1  that determines the quantization resolutions for the subband signals by a reiterative process that applies the synthesis-filter noise-spreading model to proposed quantization resolutions, adjusts the proposed quantization resolutions, and reiterates until the one or more comparison criteria are satisfied. 
     
     
       4. A method according to  claim 3  wherein the reiterative process comprises: 
       identifying one or more subband signal components the quantization of which, according to the synthesis-filter noise-spreading model, contributes to a portion of the estimated noise levels that exceeds a corresponding portion of the desired-noise spectrum;  
       selecting the subband signal component the quantization of which, according to the synthesis-filter noise-spreading model, makes the largest contribution to the portion of the estimated noise levels that exceeds the corresponding portion of the desired noise spectrum; and  
       adjusting the respective proposed quantization resolution for the selected subband signal component.  
     
     
       5. A method according to  claim 3  wherein the reiterative process comprises: 
       identifying one or more subband signal components the quantization of which, according to the synthesis-filter noise-spreading model, contributes to a portion of the estimated noise levels that exceeds a corresponding portion of the desired-noise spectrum;  
       selecting the subband signal component the quantization of which, according to the synthesis-filter noise-spreading model, makes the largest contribution to the portion of the estimated noise levels that exceeds the corresponding portion of the desired noise spectrum;  
       increasing the proposed quantization resolution for the selected subband signal component by a first amount, and increasing the proposed quantization resolution for one or more other subband signal components that are neighbors to the selected subband signal component by a second amount that is less than the first amount.  
     
     
       6. A method according to  claim 3  wherein the reiterative process comprises: 
       applying the synthesis-filter noise-spreading model to obtain estimated individual noise contributions for individual subband signal components; and  
       increasing the proposed quantization resolution for those individual subband signal components making estimated individual noise contributions that exceed the desired noise spectrum.  
     
     
       7. A method according to  claim 1  wherein the synthesis-filter noise-spreading model is a function that expresses synthesis filter output noise at a respective frequency as a function of synthesis filter input noise at a plurality of frequencies. 
     
     
       8. A method according to  claim 1  that comprises quantizing the subband signals according to the determined quantization resolutions and assembling the quantized subband signals into an encoded signal. 
     
     
       9. A method according to  claim 1  that comprises obtaining the quantized subband signals from an encoded signal and dequantizing the quantized subband signals according to the determined quantization resolutions. 
     
     
       10. An apparatus for establishing quantization resolutions for quantizing subband signals obtained from analysis filters that are applied to an input signal, wherein an output signal that is a replica of the input signal is to be obtained by applying synthesis filters to dequantized representations of the quantized subband signals and by applying an overlap-add process to blocks of information obtained from the synthesis filters, the apparatus comprising: 
       an input terminal that receives the input signal; and  
       one or more processing circuits coupled to the input terminal for generating a desired noise spectrum in response to the input signal, and for determining the quantization resolutions for the subband signals by applying a synthesis-filter noise-spreading model to obtain estimated noise levels in subbands of the output signal obtained from the synthesis filters, wherein the synthesis-filter noise-spreading model represents noise-spreading characteristics of the synthesis filters and accounts for effects of the overlap-add process, and wherein the quantization resolutions are determined such that a comparison of the desired-noise spectrum with the estimated noise levels satisfies one or more comparison criteria.  
     
     
       11. An apparatus according to  claim 10  wherein the one or more processing circuits determine the quantization resolutions for the subband signals by performing a process that applies the synthesis-filter noise-spreading model to proposed quantization resolutions and adjusts the proposed quantization resolutions by a predefined amount of compensation. 
     
     
       12. An apparatus according to  claim 10  wherein the one or more processing circuits determine the quantization resolutions for the subband signals by performing a reiterative process that applies the synthesis-filter noise-spreading model to proposed quantization resolutions, adjusts the proposed quantization resolutions, and reiterates until the one or more comparison criteria are satisfied. 
     
     
       13. An apparatus according to  claim 12  wherein the reiterative process comprises: 
       identifying one or more subband signal components the quantization of which, according to the synthesis-filter noise-spreading model, contributes to a portion of the estimated noise levels that exceeds a corresponding portion of the desired-noise spectrum;  
       selecting the subband signal component the quantization of which, according to the synthesis-filter noise-spreading model, makes the largest contribution to the portion of the estimated noise levels that exceeds the corresponding portion of the desired noise spectrum; and  
       adjusting the respective proposed quantization resolution for the selected subband signal component.  
     
     
       14. An apparatus according to  claim 12  wherein the reiterative process comprises: 
       identifying one or more subband signal components the quantization of which, according to the synthesis-filter noise-spreading model, contributes to a portion of the estimated noise levels that exceeds a corresponding portion of the desired-noise spectrum;  
       selecting the subband signal component the quantization of which, according to the synthesis-filter noise-spreading model, makes the largest contribution to the portion of the estimated noise levels that exceeds the corresponding portion of the desired noise spectrum;  
       increasing the proposed quantization resolution for the selected subband signal component by a first amount, and increasing the proposed quantization resolution for one or more other subband signal components that are neighbors to the selected subband signal component by a second amount that is less than the first amount.  
     
     
       15. An apparatus according to  claim 12  wherein the reiterative process comprises: 
       applying the synthesis-filter noise-spreading model to obtain estimated individual noise contributions for individual subband signal components; and  
       increasing the proposed quantization resolution for those individual subband signal components making estimated individual noise contributions that exceed the desired noise spectrum.  
     
     
       16. An apparatus according to  claim 10  wherein the one or more processing circuits apply the synthesis-filter noise-spreading model that is a function that expresses synthesis filter output noise at a respective frequency as a function of synthesis filter input noise at a plurality of frequencies. 
     
     
       17. An apparatus according to  claim 10  wherein the one or more processing circuits generate an encoded representation of the input signal by quantizing the subband signals according to the determined quantization resolutions and assembling the quantized subband signals into the encoded signal. 
     
     
       18. An apparatus according to  claim 10  wherein the one or more processing circuits decode an encoded signal conveying the quantized subband signals by extracting the quantized subband signals from the encoded signal and dequantizing the quantized subband signals according to the determined quantization resolutions. 
     
     
       19. A receiver that receives and decodes a signal conveying encoded information and generates an output signal by applying synthesis filters to dequantized representations of quantized components of subband signals and by applying an overlap-add process to blocks of information obtained from the synthesis filters, wherein the encoded information comprises: 
       (1) signal information that represents the quantized components of subband signals generated by an encoder that applies analysis filters to an input signal; and  
       (2) control information that represents quantizing resolutions of the quantized subband signal components, wherein the quantizing resolutions are determined in the encoder by  
       (a) generating a desired noise spectrum in response to the input signal; and  
       (b) applying a synthesis-filter noise-spreading model to obtain estimated noise levels in subbands of an output signal obtained from synthesis filters, wherein the synthesis-filter noise-spreading model represents noise-spreading characteristics of the synthesis filters and the overlap-add process, and wherein the quantization resolutions are determined such that a comparison of the desired-noise spectrum with the estimated noise levels satisfies one or more comparison criteria;  
       and wherein the receiver comprises: 
       (1) an input coupled to the signal conveying the encoded information;  
       (2) one or more processing circuits coupled to the input that  
       (a) extract the signal information and the control information from the encoded information and obtain therefrom the quantized subband signal components and the quantizing resolutions of the quantized subband signal components;  
       (b) dequantize the quantized subband signal components according to the quantizing resolutions to obtain dequantized subband signals; and  
       (c) apply the synthesis filters to the dequantized subband signals and apply the overlap-add process to blocks of information obtained from the synthesis filters to generate an output signal, wherein quantizing noise in the subband signals is spread by the synthesis filters to produce noise levels in subbands of the output signal that substantially satisfy the one or more comparison criteria with the desired-noise spectrum; and  
       (3) an output coupled to the one or more processing circuits that conveys the output signal.  
     
     
       20. A receiver according to  claim 19  wherein the one or more comparison criteria is that noise levels in subbands of the output signal are offset from the desired-noise spectrum by amounts that are substantially constant. 
     
     
       21. A medium conveying encoded information to be decoded by applying synthesis filters to dequantized representations of quantized components of subband signals and by applying an overlap-add process to blocks of information obtained from the synthesis filters, wherein the encoded information comprises: 
       (1) signal information that represents the quantized components of subband signals generated by applying analysis filters to an input signal; and  
       (2) control information that represents quantizing resolutions of the quantized subband signal components, wherein the quantizing resolutions are determined by  
       (a) generating a desired noise spectrum in response to the input signal; and  
       (b) applying a synthesis-filter noise-spreading model to obtain estimated noise levels in subbands of an output signal obtained from synthesis filters, wherein the synthesis-filter noise-spreading model represents noise-spreading characteristics of the synthesis filters and accounts for effects of the overlap-add process, and wherein the quantization resolutions are determined such that a comparison of the desired-noise spectrum with the estimated noise levels satisfies one or more comparison criteria.  
     
     
       22. A medium according to  claim 21  wherein the one or more comparison criteria is that noise levels in subbands of the output signal are offset from the desired-noise spectrum by amounts that are substantially constant. 
     
     
       23. A medium readable by a device embodying a program of instructions for execution by the device to perform a method for establishing quantization resolutions for quantizing subband signals obtained from analysis filters that are applied to an input signal, wherein an output signal that is a replica of the input signal is to be obtained by applying synthesis filters to dequantized representations of the quantized subband signals and by applying an overlap-add process to blocks of information obtained from the synthesis filters, the method comprising: 
       generating a desired noise spectrum in response to the input signal; and  
       determining the quantization resolutions for the subband signals by applying a synthesis-filter noise-spreading model to obtain estimated noise levels in subbands of the output signal obtained from the synthesis filters, wherein the synthesis-filter noise-spreading model represents noise-spreading characteristics of the synthesis filters and accounts for effects of the overlap-add process, and wherein the quantization resolutions are determined such that a comparison of the desired-noise spectrum with the estimated noise levels satisfies one or more comparison criteria.  
     
     
       24. A medium according to  claim 23  that determines the quantization resolutions for the subband signals by a process that applies the synthesis-filter noise-spreading model to proposed quantization resolutions and adjusts the proposed quantization resolutions by a predefined amount of compensation. 
     
     
       25. A medium according to  claim 23  that determines the quantization resolutions for the subband signals by a reiterative process that applies the synthesis-filter noise-spreading model to proposed quantization resolutions, adjusts the proposed quantization resolutions, and reiterates until the one or more comparison criteria are satisfied. 
     
     
       26. A medium according to  claim 25  wherein the reiterative process comprises: 
       identifying one or more subband signal components the quantization of which, according to the synthesis-filter noise-spreading model, contributes to a portion of the estimated noise levels that exceeds a corresponding portion of the desired-noise spectrum;  
       selecting the subband signal component the quantization of which, according to the synthesis-filter noise-spreading model, makes the largest contribution to the portion of the estimated noise levels that exceeds the corresponding portion of the desired noise spectrum; and  
       adjusting the respective proposed quantization resolution for the selected subband signal component.  
     
     
       27. A medium according to  claim 25  wherein the reiterative process comprises: 
       identifying one or more subband signal components the quantization of which, according to the synthesis-filter noise-spreading model, contributes to a portion of the estimated noise levels that exceeds a corresponding portion of the desired-noise spectrum;  
       selecting the subband signal component the quantization of which, according to the synthesis-filter noise-spreading model, makes the largest contribution to the portion of the estimated noise levels that exceeds the corresponding portion of the desired noise spectrum;  
       increasing the proposed quantization resolution for the selected subband signal component by a first amount, and increasing the proposed quantization resolution for one or more other subband signal components that are neighbors to the selected subband signal component by a second amount that is less than the first amount.  
     
     
       28. A medium according to  claim 25  wherein the reiterative process comprises: 
       applying the synthesis-filter noise-spreading model to obtain estimated individual noise contributions for individual subband signal components; and  
       increasing the proposed quantization resolution for those individual subband signal components making estimated individual noise contributions that exceed the desired noise spectrum.  
     
     
       29. A medium according to  claim 23  wherein the synthesis-filter noise-spreading model is a function that expresses synthesis filter output noise at a respective frequency as a function of synthesis filter input noise at a plurality of frequencies. 
     
     
       30. A medium according to  claim 23  wherein the method comprises quantizing the subband signals according to the determined quantization resolutions and assembling the quantized subband signals into an encoded signal. 
     
     
       31. A medium according to  claim 23  wherein the method comprises obtaining the quantized subband signals from an encoded signal and dequantizing the quantized subband signals according to the determined quantization resolutions.